In internal combustion engines, mainly in the overhead valve models, the valves are cyclically opened and closed during the rotation of the motor through one or more camshafts. These camshafts move the valves by a transmission as shown, for example, in the patents to Papenguth, U.S. Pat. No. 2,818,050, issued Dec. 31, 1957, and Abell U.S. Pat. No. 3,448,730, issued June 10, 1969, and comprising a cam, a lifter or tappet, a pushrod, a rocker arm which oscillates about its fulcrum, and an intake or exhaust valve.
In order to provide an automatic compensation of the varying lengths of the above-indicated kinematic transmission due to thermal variation during the functioning of the motor, there is in widespread use as lifter an hydraulic valve lifter, operating between the cam on the camshaft and the pushrod. Its constitution may vary, but it is basically composed of a cup-shaped cylindrical body that houses all the other parts of the lifter, a pushrod socket, a hollow plunger of cylindrical shape and a check valve mounted at the lower extremity of the plunger which may be in the form of a ball acted on by a spring within a spring holder. These three parts are assembled in the bottom of the plunger and they allow the free flow of oil from an oil reservoir within the plunger to a reservoir at the bottom of the body, but they prevent the opposite flow. This assembly is pushed up in the direction from cam to pushrod by a spring and is retained within the body by a snap ring.
In this hydraulic valve lifter, the relative position of all parts in contact with the pushrod (all except the body and snap-ring) and the position of the body, are variable and infinitely adjustable. To effect this result, the lifter should be fully filled with oil, supplied by the motor, which enters through the body cross-hole to an annular groove, in the interior surface of the body, from such groove to a groove on the outer wall of the plunger, the dimension of these two grooves being so calculated that there is always communication between them in any relative position of the plunger and body, and through the cross-hole of the plunger into the oil reservoir of the latter. This oil flows through the check valve into the oil reservoir in the body every time the spring pushes the plunger in the direction toward the pushrod, which happens every time there is a clearance in the kinematic transmission and the oil reservoir in the body is not fully filled with oil. It is obvious that in a short time after start of the engine, the oil reservoir in the bottom of the body will be fully filled with oil.
For a correct functioning of the lifter, it is necessary that the oil should not easily get out of the reservoir in the body, but it is necessary that some oil should leak out, to allow for the change of length of the valve train as required. The well-known solution consists in assembling the body and plunger with a very small clearance (about 0.006 millimeters on the diameter) to allow for only a small leakage of oil between them, through such clearance. To control this leakage value, it is usual to establish two experimental times (t.sub.0 and t.sub.1) such that, when a constant force F in the direction pushrod to cam is applied on the pushrod of a fully filled hydraulic valve lifter, the plunger must take a certain amount of time t, to move along a predetermined travel; this time t is known as "leakdown time" and it should be such that t.sub.0 .ltoreq.t.ltoreq.t.sub.1.
This test, called "leakdown test," is then a simulation of the real functioning of the motor. It is here that there arises one of the problems that my invention is intended to solve, which I shall refer to as PROBLEM 2, and will be fully explained below.
It is usual to use this hydraulic valve lifter for another purpose besides the one above indicated. In fact, for greater durability of the motor, any friction point should be carefully lubricated. So, the contact point of the pushrod in the rocker arm, the rocker arm bearing and the contact point of the rocker arm on the valve rod should be carefully lubricated. This lubrication was formerly effected through an independent oil gallery, but nowadays is generally made through the pushrod itself, which is a tubular member and receives the oil through the hydraulic lifter. For this purpose, the lifter has near the open end of the body an axial hole in the socket through which there flows the oil needed for the lubrication of the valve train.
It is essential that the amount of oil should be carefully controlled to avoid insufficient lubrication, on the one hand, which could cause the parts to stick together, or, on the other hand, too much oil should likewise be avoided, as this would lead to an excess of oil in the cylinder head, which would drain along the valve rod and be burnt and lost in the motor. It is here that there arises another problem that my invention seeks to solve, and which I refer to as PROBLEM 1, which will be fully explained hereinbelow.
Finally, for a complete knowledge of this mechanism it is necessary to remark that another movement exists besides the axial one, namely, between the body and the plunger. In fact, the lifters are mounted in an eccentric position on the cam, so that an eccentricity "e" always exists. In consequence, the rotation of the cam about its axis induces a rotation of the tappet about its axis too. This rotation has for its object to avoid a constant wearing point of contact between the cam and tappet. With this construction, the contact point rotates about the axis, leading to a more lasting tappet.